When the results of the 2001 census were published, we were asked to believe that 72% of people in England and Wales were Christians. But in the same year, the British Social Attitudes (BSA) survey measured only 51.2% of the population as Christian: a difference of a fifth of the population. Subsequent surveys agree with the BSA, and its 2009 survey indicated that more than 50% of the population are now in fact non-religious.

Those who take the time to investigate the census results see clearly that they are ridiculous. If we believed them, we would believe that there are more Jedis in England and Wales than Jews, Buddhists or Sikhs. We would believe – contrary to government research that showed 65% of 12- to 18-year-olds were not religious – that in fact 62% of them (along with 58% of under-4-year-olds) were Christian.

The reasons why the data from the 2001 census was so aberrant are simple and well known. They mostly have to do with the fact that the question is a closed and leading one: "What is your religion?" This question is demonstrated to produce a much higher number of "religious" responses than non-presumptuous questions such as: "Do you have a religion?" and much higher than questions that ask about belief or practice. Faced with the closed and leading census question, people who do not believe in God, and who, if asked: "Are you religious?" would say "No", nonetheless tick "Christian" or "Sikh" or whatever.

Perhaps this would be tolerable if the census data on religion was accepted as measuring nothing more than a weak form of cultural affiliation rather than as a proxy for strong religious belief, and only used with this in mind. But the results from the forthcoming census will not just give us an interesting overview of the demographic of England and Wales for academics to critique when the results are released and for our descendants to pick over in future centuries. They will constitute a basis for policymaking over the coming years. Over the past decade the census data has been repeatedly misused. Its figures have been cited in parliament as evidence that faith is on the increase; that greater public resources should be granted to religious organisations; that the state should fund yet more faith schools. Major public policy developments have occurred and resources allocated on the back of these erroneous numbers.

The British Humanist Association worked with the Office for National Statistics for more than two years trying to secure a question in the 2011 census that would give more meaningful data. The ONS refused to change the question (though it did pledge to give guidance after the census on the ways in which data should and should not be used) and so we shifted the focus of our activity.

Now, our census campaign is using local leafleting, advertising and online activity to raise awareness of the issues surrounding the census and encourage people who have thought about the issues and who are not religious to declare that fact. Whether we like it or not, whether we approve of the question or not, how we answer the religion question in this year's census will have profound consequences for our future and we should all answer it with great thought and care.